I got it to build by downgrading vala from 38.5-1 to 0.38.4-1 (you only need 38.4-1 to build you can update after that). This will be a workaround until i can figure out a real fix.
This work around also fixes diorite building issues.

@mtorromeo
You are right. thanks for the quick sed patch for patches I thought it was VA-API just by a quick glance but it was VALA related.
@GregoD
Until i get the pkgbuilds updated as noted in the pined comments a quick workaround for people looking to get it to build is do downgrade vala to 0.36 (although i do expect working builds at the end of the day) Ok maybe Tomorrow morning. Webkit2gtk takes decades to build in my buildbox